Once mounted,

Once mounted, place your palms on the pommel and push yourself up out
of the saddle, at the same spreading your legs in a wide "V". Lower
your seat back into the saddle with your legs still stretched and feel
the difference in your seat.

Next grasp the pommel (or a strap running through the "D" rings if
that's more comfortable) and pull yourself even further down and
forward into the saddle. Now you're ready to sit the trot, in
accordance with the advice given in other posts here.

At first aim for only a few strides before repositioning yourself.
Gradually add more strides, but always stop with the first bounce and
regroup. This will be easier on both you and the horse.

Stop as soon as you feel yourself starting to bounce. Once you begin
to bump with every stride your horse's back will stiffen in self
defense and the bouncing just gets worse.

Also, if you can borrow a horse who works through his back at the
trot, so that the motion is more longitudinal than up and down, you'll
find your "eureka" moment will come much earlier and more easily.